Summary of work: Somatic hypermutation of variable (V) genes, which encode a portion of immunoglobulin molecules, occurs at a frequency that is a million times greater than mutation in other genes. The molecular mechanism that introduces these mutations is unknown. The project has three aims. The first goal is to study hypermutation in mice deficient for mismatch repair enzymes to see if the frequency and pattern of mutation is different from wildtype mice. In mice deficient for one of the repair proteins, PMS2, the frequency of mutation in V genes was similar to wildtype mice, but the pattern was altered. The data suggest that (1) tandem mutations are generated at a high frequency by a polymerase during a single event, and (2) mutations are introduced into both strands of DNA and then preferentially removed from one strand during mismatch repair. The second goal is to identify DNA sequences around the V gene that activate the hypermutation mechanism by forming secondary structures. Using biochemical techniques, a stable cruciform was found in plasmids containing the V gene, but not in plasmids with the nonmutable constant gene, suggesting that this structure may be a substrate for hypermutation. The third goal is to analyze hypermutation in V genes from old and young humans to determine if the frequency or pattern of mutation changes with age. The results will indicate if there is an age-related decline in the hypermutation mechanism.